Recently, standardization organizations such as 3GPP (3rd Generation Partnership Project), 3GPP2 (3rd Generation Partnership 2), and IEEE802.16, have been studying new standards for radio communication systems. As successor systems to the third generation (3rd Generation) cellular system, for example, a next generation cellular system (or 3.9G cellular system) such as LTE (Long Term Evolution: standard title “Evolved Universal Terrestrial Radio Access (E-UTRA)”) and UMB (Ultra Mobile Broadband) have been examined. Additionally, the IMT-Advanced system (or the 4G cellular system), regarded as an advancement to the 3.9G cellular system, has been examined.
Some of the foregoing radio communication systems adopt the orthogonal frequency division multiple access (OFDMA) standard. The LTE, UMB, and WiMax (Worldwide Interoperability for Microwave Access), for example, adopt the OFDMA standard. Additionally, the LTE-Advanced and IEEE802.16m, which have been currently examined for the 4G cellular system, adopt the OFDMA standard.
The radio communication system adopting the OFDMA standard (hereinafter, referred to as the “OFDMA system”) is able to allocate a plurality of subcarriers, available in frequency bands, to mobile terminals and change the allocation content over time. Therefore, the OFDMA system is able to flexibly allocate two-dimensional radio resources, consisting of frequency components and temporal components, to mobile terminals.
Since frequency resources securing good quality may vary depending on mobile terminals in the frequency selective phasing environment, so that it is necessary to allocate frequency resources securing good quality to mobile terminals in accordance with the frequency scheduling. The frequency scheduling improves throughputs of mobile terminals, whereby it is expected that the entire throughput of the OFDMA system is improved.
The frequency scheduling determines frequency resources allocated to mobile terminals based on qualities of frequency resources in available frequency bands. In general, reference signals included in frequency resources are used to secure quality of frequency resources. The downlink (i.e. a communication link in a direction from a radio base station to a mobile terminal) allows for transmission of reference signals via all the frequency resources in available frequency bands with a radio base station, so that a mobile station demodulates reference signals included in all the frequency resources, thus confirming the quality of all the frequency resources in available frequency bands.
On the other hand, the uplink (i.e. a communication link in a direction from a mobile terminal to a radio base station) allows for utilization of reference signals called sounding reference signals (SRS), for example, disclosed in Non-Patent Document 1. Since the SRS can be transmitted using the entire range of available frequency bands, it is possible to confirm uplink frequency characteristics (i.e. quality per each frequency resource). Non-Patent Document 2 discloses a technology for fixing radio resources used for SRS transmission per each mobile terminal based on the assumption that the number of mobile terminals is fixed.